"Radio signal controlled clocks" are clocks which receive and decode time information broadcast on specified radio frequencies. These clocks provide a reliable time source that is known to be synchronized with other such clocks--and thus can be used to coordinate activities in various locations.
For example, traffic signal manufacturers can use radio signal controlled digital clocks to align traffic signals according to the time of day without having to connect all of the traffic signals to a single clock. Thus, a large number of similarly programmed, but not physically interconnected, traffic signals in a specified area can simultaneously, or in some other coordinated fashion, modify light intervals in accordance with the time of day.
In another example, computer services can use radio signal controlled digital clocks to coordinate the activities of computers in various locations.
The National Bureau of Standards has been broadcasting time information on standard frequencies for many years from stations in Ft. Collins, Colo. and Kauai, Hi. However, the signals are relatively weak and therefore are subject to noisy reception. Thus, radio signal controlled clocks may fail to lock on to the signal for long periods of time, or adopt an incorrect timebase.
The primary objective of this invention is to provide an inexpensive, highly accurate clock that is periodically updated by a received, broadcast time reference signal.
The present invention is an improved version of the OEM-10 radio controlled digital clock made by Precision Standard Time, Inc. of Fremont, Calif., as described in the patent application entitled High Precision Radio Signal Controlled Continuously Updated Digital Clock U.S. Pat. No. 4,768,178, filed Feb. 24, 1987, assigned to Precision Standard Time, Inc. U.S. Pat. No. 4,768,178 is incorporated by reference.
In particular, the present invention provides improved methods for correctly decoding broadcast time reference signals which contain noise, multipath signals, and/or fading signal levels, so that correct time information can be derived even if virtually every time reference signal is partially corrupted by noise. Using a stringent data verification algorithm would decrease the probability of decoding a bit in error, but would also decrease the probability of decoding it at all. Therefore, an objective herein is to reduce the probability of decoding errors to an acceptable minimum, while successfully deducing the correct time within a reasonable period.
Other features of the present invention include a method of determining the location of minute and second boundaries in the broadcast time reference signals using only a subset (i.e., the 100 Hertz component) of the NBS time signal, a method of collecting and making use of time data collected before the location of the minute boundaries has been determined, a method of searching for the best time reference signal (i.e., the best of several time signal carrier frequencies broadcast by NBS), and a method of providing a variable signal strength threshold which depends on the volume of noise in the received time reference signal.